Tem sample preparation

ABSTRACT

An improved method of preparing ultra-thin TEM samples that combines backside thinning with an additional cleaning step to remove surface defects on the FIB-facing substrate surface. This additional step results in the creation of a cleaned, uniform “hardmask” that controls the ultimate results of the sample thinning, and allows for reliable and robust preparation of samples having thicknesses down to the 10 nm range.

This application claims priority from U.S. patent application Ser. No.14/502,522 filed Sep. 30, 2014, which is a continuation application ofU.S. patent application Ser. No. 13/981,563 filed Aug. 16, 2013, whichis a 371 national stage filing from international applicationPCT/US2012/023053, filed Jan. 28, 2012, which claims priority from U.S.Provisional Application 61/437,474, filed Jan. 28, 2011, which is herebyincorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to preparation of samples for transmissionelectron microscopes and, in particular, to preparation of sampleshaving a thickness of 30 nm or less.

BACKGROUND OF THE INVENTION

As semiconductor geometries continue to shrink, manufacturesincreasingly rely on Transmission electron microscopes (TEMs) formonitoring the process, analyzing defects, and investigating interfacelayer morphology. Transmission electron microscopes (TEMs) allowobservers to see features having sizes on the order of nanometers. Incontrast to SEMs, which only image the surface of a material, TEM alsoallows analysis of the internal structure of a sample. In a TEM, a broadbeam impacts the sample and electrons that are transmitted through thesample are focused to form an image of the sample. The sample must besufficiently thin to allow many of the electrons in the primary beam totravel though the sample and exit on the opposite site.

Because a sample must be very thin for viewing with transmissionelectron microscopy (whether TEM or STEM), preparation of the sample canbe delicate, time-consuming work. The term “TEM” as used herein refersto a TEM or a STEM and references to preparing a sample for a TEM are tobe understood to also include preparing a sample for viewing on an STEM.The term “STEM” as used herein also refers to both TEM and STEM.

TEM samples are typically less than 100 nm thick, but for someapplications samples must be considerably thinner. With advancedprocesses at 30 nm and below, the sample needs to be less than 20 nm inthickness in order to avoid overlap among small scale structures.Currently thinning below 30 nm is difficult and not robust. Thicknessvariations in the sample result in sample bending, overmilling, or othercatastrophic defects. For such small samples, preparation is a criticalstep in TEM analysis that significantly determines the quality ofstructural characterization and analysis of the smallest and mostcritical structures.

Even though the information that can be discovered by TEM analysis canbe very valuable, the entire process of creating and measuring TEMsamples has historically been so labor intensive and time consuming thatit has not been practical to use this type of analysis for manufacturingprocess control. While the use of FIB methods in sample preparation hasreduced the time required to prepare samples for TEM analysis down toonly a few hours, it is not unusual to analyze 15 to 50 TEM samples froma given wafer. As a result, speed of sample preparation is a veryimportant factor in the use of TEM analysis, especially forsemiconductor process control.

A significant problem for the preparation of ultra thin (<30 nm thick)TEM samples is commonly referred to as “curtaining,” in whichnon-uniform high-density materials on the surface of an integratedcircuit produce a non-planar face on the TEM sample after thinning.Top-down thinning of a sample having these types of structural ordensity variations will cause vertical ridges to propagate from thedenser materials (i.e. metal lines) near the top of the sample (the topbeing defined as closest to the ion beam source) down the face of thecross-section, running in a direction parallel to the ion beamdirection. Curtaining is most often observed in semiconductor materialswhere multiple patterned layers of materials having a low sputteringyield blocks a faster sputtering yield material. Curtaining may also beobserved in materials exhibiting different topographic regions wherechanges in sputtering yields vary with the milling incident angle.Curtaining artifacts reduce the quality of the TEM imaging and limit theminimal useful specimen thickness. For ultra-thin TEM samples, definedherein as samples having a thickness of less than 30 nm, the twocross-section faces are obviously in very close proximity so thicknessvariations from curtaining effects can cause a sample to be unusable.FIGS. 1A and 1B show photomicrographs of thinned samples showingcurtaining on the sample faces.

In order to minimize curtaining in TEM sample preparation, it is knownto invert the samples so that the bottom of the sample (the substrate)is facing the FIB column. Because the substrate portion of the samplewill not have imbedded features such as metal lines or transistors,curtaining artifacts will not be introduced into the portion of thesample face containing the region of interest, i.e., the layers ofcircuitry on the top surface of the semiconductor. While this techniqueworks reasonably well for TEM samples having a thickness of 50 to 100nm, for ultra-thin samples having a sample thickness of 30 nm or less,even samples prepared by inverting the sample before thinning often showmilling artifacts resulting in a undesirably non-uniform sample face.

Thus, there is still a need for an improved method of TEM samplepreparation to allow the preparation of ultra-thin TEM samples.

SUMMARY OF THE INVENTION

It is an object of the invention, therefore, to provide an improvedmethod of preparing ultra-thin TEM samples. Preferred embodiments of thepresent invention combine the current backside thinning process with anadditional cleaning step to remove surface defects on the FIB-facingsubstrate surface. This additional step results in the creation of acleaned, uniform “hardmask” that controls the ultimate results of thesample thinning, and allows for reliable and robust preparation ofsamples having thicknesses down to the 10 nm range.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1A is a photomicrograph of a thinned TEM sample showing curtaining;

FIG. 1B is a photomicrograph of an inverted TEM sample where thecurtaining is outside the region of interest;

FIG. 2 is a schematic representation showing the location of a TEMsample to be extracted within a larger substrate;

FIGS. 3A-3D are photomicrograph images showing the sequence of a typicalin-situ lift out of a chunk-type TEM;

FIGS. 4A-4C are photomicrograph images showing the sequence of mountingan inverted TEM sample on a TEM sample grid;

FIG. 5 is a schematic representation showing the inverted TEM samplemounted on a TEM sample grid;

FIG. 6 is a schematic representation showing the inverted TEM sampletilted so that the FIB can be used to clean the substrate backsidesurface;

FIG. 7A shows a top down photomicrograph of a substrate backside surfacebefore cleaning;

FIG. 7B is a photomicrograph of a side view of the sample of FIG. 7Ashowing the location of the FIB cut line;

FIG. 7C is a top down photomicrograph of the substrate backside surfaceof FIG. 7A after the FIB cleaning cut and with substantially all of thenon-uniformities on the substrate backside surface removed;

FIGS. 8A and 8B show the process of thinning a sample after the FIB hasbeen used to clean the substrate backside surface;

FIG. 9 shows a photomicrograph of a TEM sample ˜20 nm thick producedwithout the substrate backside cleaning step;

FIG. 10 shows a photomicrograph of a TEM sample <15 nm thick producedusing the additional substrate backside cleaning step;

FIG. 11 is a flowchart showing the steps of creating an ultra thin TEMsample according to a preferred embodiment of the present invention; and

FIG. 12 shows a typical dual beam FIB/STEM system that could be used toimplement the present invention.

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are directed at a novelmethod of preparing ultra-thin TEM samples. Preferred embodiments of thepresent invention combine the current backside thinning process with anadditional cleaning step to remove surface defects on the FIB-facingsubstrate surface. During the typical TEM sample extraction process, thebottom surface of the sample will accumulate redeposited material fromthe sample extraction ion milling process. Non-uniformities on thebottom surface can also result from milling artifacts accrued during thebulk material removal process. Applicants have discovered that thesematerial or topographical variations on the surface of the substrateside of the sample have a significant effect upon the TEM samplethinning process. These types of surface variations propagate throughthe milling process as the TEM sample (also referred to as a lamella) isthinned and result in sidewall non-uniformities that limit the minimumthickness to which the sample may be thinned. Applicants have discoveredthat these types of non-uniformities have resulted in the historicallylimited success of thinning samples thinner than 30 nm.

Preferred embodiments of the present invention introduce an additionalstep to the preparation of a backside thinned TEM sample in which theFIB-facing substrate surface is “cleaned” with the FIB to form a uniformbackside substrate surface. As described in greater detail below, theFIB can be used to mill away the “dirty” substrate surface, forming acleaned, uniformly flat substrate surface that functions as a sort of“hard mask” during TEM sample thinning in that it protects the region ofinterest below (when the sample is inverted) and it controls thecreation of a smooth, flat TEM sample face. The additional step offorming the hard mask, while undesirably increasing the amount of timerequired to prepare a TEM sample, allows for reliable and robustpreparation of samples having thicknesses down to the 10 nm range. Thereliability of the methods described herein make the methods especiallysuitable for automated sample preparation.

While it is desirable that the flat surface formed by milling away aportion of the sample backside be as uniformly flat as possible to avoidintroducing irregularities or artifacts during lamella thinning, skilledpersons will recognize that this desire must be weighed against theincreased time and expense of producing TEM samples. Applicants havediscovered, as described in greater detail below, that using an ionbeam, for example a 30 kV gallium ion beam, to mill away or “cut off” aportion of the bottom of the extracted sample typically produces asufficiently smooth surface to prepare ultra-thin TEM samples. As usedherein, the phrase “ultra-thin TEM samples” will be used to refer tosamples where the entire lamella or a portion of the lamella (such as a“window” large enough for TEM imaging) is thinned to a thickness of 30nm or less.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable.

FIG. 11 is a flowchart showing the steps of creating an ultra thin TEMsample according to a preferred embodiment of the present invention.Various steps in the process are shown in FIGS. 2 through 10.

First, in step 201, a substrate such as a semiconductor wafer is loadedinto a Dual Beam FIB/STEM system having both a FIB column and a SEMcolumn Referring also to FIG. 12, the typical dual-beam system 302configuration is an electron column 304 having a vertical axis with anion column 306 having an axis tilted with respect to the vertical(usually at a tilt of approximately 52 degrees). Wafers are preferablytransferred by way of a multi-wafer carrier and auto-loading robot (notshown), as in well known in the art, although wafers can also betransferred manually.

In step 202, the location of a sample (containing a feature of interest)to be extracted from a substrate is determined. For example, thesubstrate may be a semiconductor wafer or portion thereof and theportion to be extracted may include a portion of an integrated circuitthat is to be observed using the TEM. The location of the sample can bedetermined using a variety of methods known in the prior art. Forexample, the sample location can be located using coordinates based onCAD data for the semiconductor wafer. Lamella sites on the wafer surfacecan also be located automatically using image recognition software.Suitable image recognition software is available, for example, fromCognex Corporation of Natick, Mass. Image recognition software can be“trained” to locate the desired lamella locations by using sample imagesof similar features or by using geometric information from CAD data.

FIG. 2 is a schematic representation showing the location of the sample20 to be extracted within a larger substrate 21. For convenience, theupper portion of the sample oriented toward the substrate surfaceclosest to the charged particle beams will be referred to herein as the“top” of the sample 26, even after the sample is removed from thesubstrate and its orientation changed. Likewise, the lower portion ofthe sample oriented away from the substrate surface closest to thecharged particle beams and toward the bulk substrate material will bereferred to herein as the “bottom” of the sample 27, even after thesample is removed from the substrate and its orientation changed. Thevertical axis of the sample is shown by dashed line 62.

In step 204, the sample 20 is completely or partially separated from thesubstrate 21 by milling with a focused ion beam. As discussed below,this step can be accomplished by using a dual beam FIB/SEM system suchas the Helios1200 Expida™ 1255 DualBeam™ System, available from FEICompany of Hillsboro, Oreg., the assignee of the present invention.Next, a microprobe tip 23 is attached to the sample by FIB-inducedchemical vapor deposition. In the case of only partially separatedsamples, the sample is then completely freed by further FIB milling.This process typically results in a wedge-shaped sample 20, which isapproximately 10×5×5 μm in size. In step 206, the sample 20 is thenlifted free of the substrate 21 by the attached microprobe 23. Thissequence is shown sequentially in the photomicrographs of FIGS. 3A to3D.

In step 208, the sample is then transported by the attached microprobeto a TEM sample holder 24 as shown in FIG. 4A. Sample holder 24 cancomprise, for example, a TEM finger grid. Referring also to FIG. 12, theTEM sample holder is preferably mounted vertically onto a stage so thatthe vertical axis 64 of TEM sample holder 24 is perpendicular to theplane of the sample stage surface. In the embodiment shown in FIG. 4A,the vertical axis 62 of the sample is substantially parallel to thevertical axis 64 of the TEM sample holder 24. Although otherorientations are possible, this orientation is described herein forsimplicity.

In step 210, the sample is then inverted by rotating the microprobe sothat the substrate side 25 of the sample is facing up (also referred toas the backside). In other words, the sample is rotated around an axisperpendicular to the sample's vertical axis in order to invert the topand bottom sides of the sample. FIG. 4B shows a top down view of thesample in close proximity to the TEM sample holder. In step 212, thesample 20 is attached to the sample holder 24 (again with FIB-inducedCVD) and then end of the sample where the microprobe 23 is attached iscut free, as shown in FIG. 4C. FIG. 5 is a schematic representationshowing the inverted TEM sample mounted on a TEM sample grid.

As shown in photomicrographs 4B and 4C, the substrate backside (facingup after the sample is inverted) has substantial non-uniformitiesresulting from redeposition or from milling artifacts. This “dirty”substrate face has been rotated up by inverting the sample and faces theFIB. Applicants have discovered that these non-uniformities on thesubstrate backside have a significant impact upon the ultimate result ofthinning a sample to a thickness of 30 nm or less. Thus, in step 214 thesample is tilted so that the FIB will be as perpendicular as possible tothe substrate backside surface given the stage/manipulator used in theFIB system. This is shown schematically in FIG. 6. In the system shownin FIG. 6, the TEM sample holder 24 has been tilted from 0 degrees(vertical) to roughly 90 degrees (horizontal). In step 216, the FIB isthen used to mill away the substrate backside surface at the angleindicated by cut line 28. The actual amount of material to be removedwill depend upon the surface irregularities present.

If the FIB is at 52 degrees with respect to the vertical axis of the SEMcolumn, that would make the cut line 28 of the FIB at an angle ofapproximately 38 degrees relative to the axis 64 of the sample holder 24(and thus to the vertical axis 62 of the sample). While the FIB cut line28 in FIG. 6 is not perpendicular to the substrate backside surface, acleaning cut at this angle would still be sufficient to remove thesample surface non-uniformities described above and produce a uniformlyflat backside surface. The use of the term “uniformly flat” is intendedto convey that after milling the backside surface is planar and freefrom any significant irregularities. The term is not intended to implythat the flat surface is necessarily perpendicular to the samplevertical axis. In fact, as described herein, the angle of the flatbackside surface in relation to the vertical axis of the sample can beanywhere within a wide range of angles, preferably anywhere from 35 to90 degrees.

The substrate backside surface can be smoothed or flattened by using theFIB at a lower operating accelerating voltage to mill away or cut offthe bottom portion of the extracted sample. This serves to remove thenon-uniformities on the surface without causing significant redepositionor milling artifacts, resulting in a post-cut clean and smooth substratebackside surface. Preferably, the backside milling is accomplishedusing, for example with a 5 kV FIB, rather than the ˜30 kV FIB typicallyused for bulk material removal. However, Applicants have discovered thatthis step is not especially sensitive to FIB beam damage and so low-kVFIB milling is not always required for cleaning the backside surface. Inother words, removing a portion of the backside surface using a 30 kVFIB will often produce a sufficiently smooth surface to greatly improvethe quality of the lamella. The desire for a smoother surface will haveto be weighed against the increased production time to determine whichapproach is optimal in a given circumstance.

FIG. 7A shows a top down photomicrograph of a substrate backside surface25 before the cleaning cut described above. FIG. 7B is a photomicrographof a side view of the sample 20 of FIG. 7A, with the location of the cutline shown by dashed line 28. And finally, FIG. 7C is a top downphotomicrograph of the substrate backside surface 25 of FIG. 7A afterthe FIB cleaning cut (using a 5 kV FIB) and with substantially all ofthe non-uniformities on surface 25 removed.

In step 218, once a more uniform substrate backside surface has beenformed, the sample can be tilted back so that substrate backside surfaceagain faces the FIB beam. Then in step 220, the sample is thinned,preferably from both sides, into an electron-transparent thin section bymilling with the ion beam as shown in FIGS. 8A-8B. The uniformFIB-facing surface produced as described above eliminates cross-sectionthickness variations on both sides of the TEM sample. FIG. 9 shows aphotomicrograph of a TEM sample ˜20 nm thick produced without thesubstrate backside cleaning step. As shown in FIG. 9, there aresubstantial thickness variations in this TEM sample. FIG. 10, however,shows a cross-section photomicrograph of a TEM sample <15 nm thickproduced using the additional substrate backside-cleaning step. Thethickness variations seen in the sample of FIG. 9 are not present inFIG. 10, as shown by the lack of curtaining on the sample in FIG. 10,even though the sample in FIG. 10 is at least 25% thinner than thesample of FIG. 9.

Finally, in step 222, the sample 20 can be imaged using the electronbeam and TEM detector, either in the dual beam system or after transportto a separate TEM instrument.

FIG. 12 depicts one embodiment of an exemplary dual beam SEM/FIB system302 that is equipped to carry out a method according to the presentinvention. As discussed above, embodiments of the present invention canbe used in a wide variety of applications where a material is depositedonto a target surface of a substrate, including the preparation of TEMsamples from vitrified biological samples. Preparation and analysis ofsuch a sample is typically performed in a dual beam electronbeam/focused ion beam system such as the one now described. FIG. 17depicts an exemplary dual beam system 302 that can be used to carry outembodiments of the invention. Suitable dual beam systems arecommercially available, for example, from FEI Company, Hillsboro, Oreg.,the assignee of the present application. While an example of suitablehardware is provided below, the invention is not limited to beingimplemented in any particular type of hardware.

Dual beam system 302 has a vertically mounted electron beam column 304and a focused ion beam (FIB) column 306 mounted at an angle ofapproximately 52 degrees from the vertical on an evacuable specimenchamber 308. The specimen chamber may be evacuated by pump system 309,which typically includes one or more, or a combination of, aturbo-molecular pump, oil diffusion pumps, ion getter pumps, scrollpumps, or other known pumping means.

The electron beam column 304 includes an electron source 310, such as aSchottky emitter or a cold field emitter, for producing electrons, andelectron-optical lenses 312 and 314 forming a finely focused beam ofelectrons 316. Electron source 310 is typically maintained at anelectrical potential of between 500 V and 30 kV above the electricalpotential of a work piece 318, which is typically maintained at groundpotential.

Thus, electrons impact the work piece 318 at landing energies ofapproximately 500 eV to 30 keV. A negative electrical potential can beapplied to the work piece to reduce the landing energy of the electrons,which reduces the interaction volume of the electrons with the workpiece surface, thereby reducing the size of the nucleation site. Workpiece 318 may comprise, for example, a semiconductor device,microelectromechanical system (MEMS), or a lithography mask. The impactpoint of the beam of electrons 316 can be positioned on and scanned overthe surface of a work piece 318 by means of deflection coils 320.Operation of lenses 312 and 314 and deflection coils 320 is controlledby scanning electron microscope power supply and control unit 322.Lenses and deflection unit may use electric fields, magnetic fields, ora combination thereof.

Work piece 318 is on movable stage 324 within specimen chamber 308.Stage 324 can preferably move in a horizontal plane (X and Y axes) andvertically (Z axis) and can tilt approximately sixty (60) degrees androtate about the Z axis. A door 327 can be opened for inserting workpiece 318 onto X-Y-Z stage 324 and also for servicing an internal gassupply reservoir (not shown), if one is used. The door is interlocked sothat it cannot be opened if specimen chamber 308 is evacuated.

Mounted on the vacuum chamber are multiple gas injection systems (GIS)330 (two shown). Each GIS comprises a reservoir (not shown) for holdingthe precursor or activation materials and a needle 332 for directing thegas to the surface of the work piece. Each GIS further comprises means334 for regulating the supply of precursor material to the work piece.In this example the regulating means are depicted as an adjustablevalve, but the regulating means could also comprise, for example, aregulated heater for heating the precursor material to control its vaporpressure.

When the electrons in the electron beam 316 strike work piece 318,secondary electrons, backscattered electrons, and Auger electrons areemitted and can be detected to form an image or to determine informationabout the work piece. Secondary electrons, for example, are detected bysecondary electron detector 336, such as an Everhart-Thornley detector,or a semiconductor detector device capable of detecting low energyelectrons. STEM detector 362, located beneath the TEM sample holder 24and the stage 324, can collect electrons that are transmitted through asample mounted on the TEM sample holder 24. Signals from the detectors336, 362 are provided to a system controller 338. Said controller 338also controls the deflector signals, lenses, electron source, GIS, stageand pump, and other items of the instrument. Monitor 340 is used todisplay user controls and an image of the work piece using the signal.

The chamber 308 is evacuated by pump system 309 under the control ofvacuum controller 341. The vacuum system provides within chamber 308 avacuum of approximately 3×10-6 mbar. When a suitable precursor oractivator gas is introduced onto the sample surface, the chamberbackground pressure may rise, typically to about 5×10-5 mbar.

Focused ion beam column 306 comprises an upper neck portion 344 withinwhich are located an ion source 346 and a focusing column 348 includingextractor electrode 350 and an electrostatic optical system including anobjective lens 351. Ion source 346 may comprise a liquid metal galliumion source, a plasma ion source, a liquid metal alloy source, or anyother type of ion source. The axis of focusing column 348 is tilted 52degrees from the axis of the electron column. An ion beam 352 passesfrom ion source 346 through focusing column 348 and betweenelectrostatic deflectors 354 toward work piece 318.

FIB power supply and control unit 356 provides an electrical potentialat ion source 346. Ion source 346 is typically maintained at anelectrical potential of between 1 kV and 60 kV above the electricalpotential of the work piece, which is typically maintained at groundpotential. Thus, ions impact the work piece at landing energies ofapproximately 1 keV to 60 keV. FIB power supply and control unit 356 iscoupled to deflection plates 354 which can cause the ion beam to traceout a corresponding pattern on the upper surface of work piece In somesystems, the deflection plates are placed before the final lens, as iswell known in the art. Beam blanking electrodes (not shown) within ionbeam focusing column 348 cause ion beam 352 to impact onto blankingaperture (not shown) instead of work piece 318 when a FIB power supplyand control unit 356 applies a blanking voltage to the blankingelectrode.

The ion source 346 typically provides a beam of singly charged positivegallium ions that can be focused into a sub one-tenth micrometer widebeam at work piece 318 for modifying the work piece 318 by ion milling,enhanced etch, material deposition, or for imaging the work piece 318.

A micromanipulator 357, such as the AutoProbe 200™ from Omniprobe, Inc.,Dallas, Tex., or the Model MM3A from Kleindiek Nanotechnik, Reutlingen,Germany, can precisely move objects within the vacuum chamber.Micromanipulator 357 may comprise precision electric motors 358positioned outside the vacuum chamber to provide X, Y, Z, and thetacontrol of a portion 359 positioned within the vacuum chamber. Themicromanipulator 357 can be fitted with different end effectors formanipulating small objects. In the embodiments described herein, the endeffector is a thin probe 360. As is known in the prior art, amicromanipulator (or microprobe) can be used to transfer a TEM sample(which has been freed from a substrate, typically by an ion beam) to aTEM sample holder 24 for analysis.

System controller 338 controls the operations of the various parts ofdual beam system 302. Through system controller 338, a user can causeion beam 352 or electron beam 316 to be scanned in a desired mannerthrough commands entered into a conventional user interface (not shown).Alternatively, system controller 338 may control dual beam system 302 inaccordance with programmed instructions. FIG. 3 is a schematicrepresentation, which does not include all the elements of a typicaldual beam system and which does not reflect the actual appearance andsize of, or the relationship between, all the elements.

According to preferred embodiments of the invention, a method ofpreparing a sample for TEM analysis, comprises:

-   -   loading a substrate into an ion beam system;    -   separating a sample from the substrate by ion beam milling;    -   extracting the sample from the substrate, said sample having a        vertical axis, a top side, and a bottom side;    -   attaching the sample to a sample holder;    -   positioning the sample holder so that the ion beam is transverse        to the vertical axis of the sample;    -   milling the bottom side of the sample to remove at least of        portion of the bottom surface of the sample to produce a        uniformly flat surface;    -   positioning the sample holder so that the bottom side of the        sample is oriented toward the ion beam source and so that the        ion beam is parallel to the vertical axis of the sample;    -   thinning the sample by directing the ion beam in a milling        pattern that thins at least a portion of the sample to a        thickness of 30 nm or less.

In some preferred embodiments, attaching the sample to a sample holdercomprises: rotating the sample about an axis perpendicular to the samplevertical axis to invert the top and bottom sides of the sample; andattaching the inverted sample to a sample holder.

In some preferred embodiments, positioning the sample holder so that theion beam is transverse to the vertical axis of the sample comprisespositioning the sample holder so that the angle between the ion beam andthe vertical axis of the sample is from 35 to 90 degrees.

In some preferred embodiments, thinning the sample by directing the ionbeam in a milling pattern comprises thinning at least a portion of thesample to a thickness of 15 nm or less.

In some preferred embodiments, milling the bottom side of the sample toremove at least of portion of the bottom surface of the sample toproduce a uniformly flat surface comprises removing at least the bottom25 nm of the sample.

In some preferred embodiments, the substrate is a semiconductor wafer orportion thereof and the sample to be extracted is a portion of anintegrated circuit that is to be observed using a TEM.

In some preferred embodiments, extracting the sample from the substratecomprises attaching a microprobe to the freed sample and extracting thesample from the substrate using the attached microprobe, and in whichattaching the sample to a sample holder comprises attaching the sampleto a sample holder and separating the microprobe from the attachedsample. In some preferred embodiments, rotating the sample about an axisperpendicular to the sample vertical axis to invert the top and bottomsides of the sample comprises inverting the sample by rotating themicroprobe so that the orientation of the top and bottom sides isreversed.

In some preferred embodiments, milling the bottom side of the sample toremove at least of portion of the bottom surface of the sample toproduce a uniformly flat surface comprises milling the bottom side ofthe sample using an ion beam having a first accelerating voltage andthen milling the bottom side of the sample at a second acceleratingvoltage, the second accelerating voltage being less than half of thefirst accelerating voltage. In some preferred embodiments, milling thebottom side of the sample to remove at least of portion of the bottomsurface of the sample to produce a uniformly flat surface comprisesmilling the bottom side of the sample using an ion beam having anaccelerating voltage of 5 kV or less. In some preferred embodiments,milling the bottom side of the sample to remove at least of portion ofthe bottom surface of the sample to produce a uniformly flat surfacecomprises milling the bottom side of the sample using an ion beam havingan accelerating voltage of 30 kV or more.

Preferred embodiments of the present invention include an apparatus forcarrying out the methods described herein. Preferred embodiments alsoinclude a non-transitory computer-readable storage medium configuredwith a computer program, where the storage medium so configured causes acomputer to control a charged particle beam system to carry out thesteps of the methods described herein.

According to preferred embodiments of the invention, a method ofpreparing an ultra thin TEM sample for TEM analysis, comprises:

-   -   loading a substrate into an ion beam system including an ion        beam source and optics for focusing an ion beam along an axis        and onto the substrate;    -   freeing a sample from the substrate by ion beam milling, said        sample having a vertical axis, a top side closest to the ion        beam source, and a bottom side opposite the ion beam source;    -   attaching a microprobe to the freed sample;    -   extracting the sample from the substrate using the attached        microprobe;    -   inverting the sample by rotating the microprobe so that the        orientation of the top and bottom sides is reversed;    -   attaching the inverted sample to a sample holder and separating        the microprobe from the attached sample;    -   positioning the sample holder so that the ion beam axis is at an        angle of 30 to 90 degrees relative to the vertical axis of the        sample;    -   removing at least 25 nm from the bottom of the sample to produce        a planar surface parallel to the ion beam axis;    -   positioning the sample holder so that the top side of the sample        is oriented opposite the ion beam source and so that the ion        beam axis is parallel to the vertical axis of the sample;    -   thinning the sample by directing the ion beam in a milling        pattern that thins at least a portion of the sample to a        thickness of 30 nm or less.

According to preferred embodiments of the invention, an apparatus forpreparing ultra-thin TEM sample comprises:

-   -   an ion beam system including an ion beam source, optics for        focusing an ion beam along an axis and onto a substrate, and a        micromanipulator for manipulating a sample; and    -   a computer-readable memory storing computer instructions, the        instructions including a program for controlling the apparatus        and causing the apparatus to carry out the steps of:    -   locating a desired sample site on the substrate;    -   separating a sample from the substrate by ion beam milling;    -   extracting the sample from the substrate, said sample having a        vertical axis, a top side, and a bottom side;    -   attaching the sample to a sample holder;    -   positioning the sample holder so that the ion beam is transverse        to the vertical axis of the sample;    -   milling the bottom side of the sample to remove at least of        portion of the bottom surface of the sample to produce a        uniformly flat surface;    -   positioning the sample so that the bottom side of the sample is        oriented toward the ion beam source and so that the ion beam is        parallel to the vertical axis of the sample; and    -   thinning the sample by directing the ion beam in a milling        pattern that thins at least a portion of the sample to a        thickness of 30 nm or less.

Although the description of the present invention above is mainlydirected at methods of preparing ultra thin TEM samples, it should berecognized that an apparatus performing the operation of such a methodwould further be within the scope of the present invention. Further, itshould be recognized that embodiments of the present invention can beimplemented via computer hardware, a combination of both hardware andsoftware, or by computer instructions stored in a non-transitorycomputer-readable memory. The methods can be implemented in computerprograms using standard programming techniques—including anon-transitory computer-readable storage medium configured with acomputer program, where the storage medium so configured causes acomputer to operate in a specific and predefined manner—according to themethods and figures described in this Specification. Each program may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits programmed for thatpurpose.

Further, methodologies may be implemented in any type of computingplatform, including but not limited to, personal computers,mini-computers, main-frames, workstations, networked or distributedcomputing environments, computer platforms separate, integral to, or incommunication with charged particle tools or other imaging devices, andthe like. Aspects of the present invention may be implemented in machinereadable code stored on a storage medium or device, whether removable orintegral to the computing platform, such as a hard disc, optical readand/or write storage mediums, RAM, ROM, and the like, so that it isreadable by a programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. Moreover, machine-readablecode, or portions thereof, may be transmitted over a wired or wirelessnetwork. The invention described herein includes these and other varioustypes of computer-readable storage media when such media containinstructions or programs for implementing the steps described above inconjunction with a microprocessor or other data processor. The inventionalso includes the computer itself when programmed according to themethods and techniques described herein.

Computer programs can be applied to input data to perform the functionsdescribed herein and thereby transform the input data to generate outputdata. The output information is applied to one or more output devicessuch as a display monitor. In preferred embodiments of the presentinvention, the transformed data represents physical and tangibleobjects, including producing a particular visual depiction of thephysical and tangible objects on a display.

Preferred embodiments of the present invention also make use of aparticle beam apparatus, such as a FIB or SEM, in order to image asample using a beam of particles. Such particles used to image a sampleinherently interact with the sample resulting in some degree of physicaltransformation. Further, throughout the present specification,discussions utilizing terms such as “calculating,” “determining,”“measuring,” “generating,” “detecting,” “forming,” or the like, alsorefer to the action and processes of a computer system, or similarelectronic device, that manipulates and transforms data represented asphysical quantities within the computer system into other data similarlyrepresented as physical quantities within the computer system or otherinformation storage, transmission or display devices.

The invention has broad applicability and can provide many benefits asdescribed and shown in the examples above. The embodiments will varygreatly depending upon the specific application, and not everyembodiment will provide all of the benefits and meet all of theobjectives that are achievable by the invention. Particle beam systemssuitable for carrying out the present invention are commerciallyavailable, for example, from FEI Company, the assignee of the presentapplication.

Although much of the previous description is directed at semiconductorwafers, the invention could be applied to any suitable substrate orsurface. Further, whenever the terms “automatic,” “automated,” orsimilar terms are used herein, those terms will be understood to includemanual initiation of the automatic or automated process or step. In thefollowing discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . . ” The term“integrated circuit” refers to a set of electronic components and theirinterconnections (internal electrical circuit elements, collectively)that are patterned on the surface of a microchip. The term“semiconductor device” refers generically to an integrated circuit (IC),which may be integral to a semiconductor wafer, singulated from a wafer,or packaged for use on a circuit board. The term “FIB” or “focused ionbeam” is used herein to refer to any collimated ion beam, including abeam focused by ion optics and shaped ion beams.

To the extent that any term is not specially defined in thisspecification, the intent is that the term is to be given its plain andordinary meaning. The accompanying drawings are intended to aid inunderstanding the present invention and, unless otherwise indicated, arenot drawn to scale.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows:
 1. A method of preparing a sample for TEM analysis,the method comprising: loading a substrate into an ion beam system;separating a sample from the substrate by ion beam milling; extractingthe sample from the substrate, said sample having a vertical axis, a topside, and a bottom side; attaching the sample to a sample holder;positioning the sample holder so that the bottom side of the sample isoriented toward the ion beam source and so that the ion beam is parallelto the vertical axis of the sample; thinning the sample by directing theion beam in a milling pattern that thins at least a portion of thesample to electron transparency; characterized by before thinning thesample, positioning the sample holder so that the ion beam is transverseto the vertical axis of the sample; milling the bottom side of thesample to remove at least of portion of the bottom surface of the sampleto produce a uniformly flat surface; and thinning the sample bydirecting the ion beam in a milling pattern that thins at least aportion of the sample to a thickness of 30 nm or less.
 2. The method ofclaim Error! Reference source not found. in which attaching the sampleto a sample holder comprises: rotating the sample about an axisperpendicular to the sample vertical axis to invert the top and bottomsides of the sample; and attaching the inverted sample to a sampleholder.
 3. The method of any of the preceding claims in whichpositioning the sample holder so that the ion beam is transverse to thevertical axis of the sample comprises positioning the sample holder sothat the angle between the ion beam and the vertical axis of the sampleis from 35 to 90 degrees.
 4. The method of any of the preceding claimsin which thinning the sample by directing the ion beam in a millingpattern comprises thinning at least a portion of the sample to athickness of 15 nm or less.
 5. The method of any of the preceding claimsin which milling the bottom side of the sample to remove at least ofportion of the bottom surface of the sample to produce a uniformly flatsurface comprises removing at least the bottom 25 nm of the sample. 6.The method of any of the preceding claims in which the substrate is asemiconductor wafer or portion thereof and the sample to be extracted isa portion of an integrated circuit that is to be observed using a TEM.7. The method of claim Error! Reference source not found. in whichextracting the sample from the substrate comprises attaching amicroprobe to the freed sample and extracting the sample from thesubstrate using the attached microprobe, and in which attaching thesample to a sample holder comprises attaching the sample to a sampleholder and separating the microprobe from the attached sample.
 8. Themethod of claim Error! Reference source not found. in which rotating thesample about an axis perpendicular to the sample vertical axis to invertthe top and bottom sides of the sample comprises inverting the sample byrotating the microprobe so that the orientation of the top and bottomsides is reversed.
 9. The method of any of the preceding claims in whichmilling the bottom side of the sample to remove at least of portion ofthe bottom surface of the sample to produce a uniformly flat surfacecomprises milling the bottom side of the sample using an ion beam havinga first accelerating voltage and then milling the bottom side of thesample at a second accelerating voltage, the second accelerating voltagebeing less than half of the first accelerating voltage.
 10. The methodof any of the preceding claims in which milling the bottom side of thesample to remove at least of portion of the bottom surface of the sampleto produce a uniformly flat surface comprises milling the bottom side ofthe sample using an ion beam having an accelerating voltage of 5 kV orless.
 11. The method of any of the preceding claims in which milling thebottom side of the sample to remove at least of portion of the bottomsurface of the sample to produce a uniformly flat surface comprisesmilling the bottom side of the sample using an ion beam having anaccelerating voltage of 30 kV or more.
 12. An apparatus for carrying outthe method of claim
 2. 13. A non-transitory computer-readable storagemedium configured with a computer program, where the storage medium soconfigured causes a computer to control a charged particle beam systemto carry out the steps of the method of claim
 2. 14. A method ofpreparing an ultra thin TEM sample for TEM analysis, the methodcomprising: loading a substrate into an ion beam system including an ionbeam source and optics for focusing an ion beam along an axis and ontothe substrate; freeing a sample from the substrate by ion beam milling,said sample having a vertical axis, a top side closest to the ion beamsource, and a bottom side opposite the ion beam source; attaching amicroprobe to the freed sample; extracting the sample from the substrateusing the attached microprobe; inverting the sample by rotating themicroprobe so that the orientation of the top and bottom sides isreversed; attaching the inverted sample to a sample holder andseparating the microprobe from the attached sample; positioning thesample holder so that the ion beam axis is at an angle of 30 to 90degrees relative to the vertical axis of the sample; removing at least25 nm from the bottom of the sample to produce a planar surface parallelto the ion beam axis; positioning the sample holder so that the top sideof the sample is oriented opposite the ion beam source and so that theion beam axis is parallel to the vertical axis of the sample; thinningthe sample by directing the ion beam in a milling pattern that thins atleast a portion of the sample to a thickness of 30 nm or less.
 15. Anapparatus for preparing ultra-thin TEM sample comprising: an ion beamsystem including an ion beam source, optics for focusing an ion beamalong an axis and onto a substrate, and a micromanipulator formanipulating a sample; and
 16. a computer-readable memory storingcomputer instructions, the instructions including a program forcontrolling the apparatus and causing the apparatus to carry out thesteps of: (i) locating a desired sample site on the substrate; (ii)separating a sample from the substrate by ion beam milling; (iii)extracting the sample from the substrate, said sample having a verticalaxis, a top side, and a bottom side; (iv) attaching the sample to asample holder; (v) positioning the sample holder so that the ion beam istransverse to the vertical axis of the sample; (vi) milling the bottomside of the sample to remove at least of portion of the bottom surfaceof the sample to produce a uniformly flat surface; (vii) positioning thesample so that the bottom side of the sample is oriented toward the ionbeam source and so that the ion beam is parallel to the vertical axis ofthe sample; and (viii) thinning the sample by directing the ion beam ina milling pattern that thins at least a portion of the sample to athickness of 30 nm or less.
 17. A method of preparing a sample for TEManalysis, the method comprising: loading a substrate into an ion beamsystem; separating a sample from the substrate by ion beam milling;extracting the sample from the substrate, said sample having a verticalaxis, a top side, and a bottom side; attaching the sample to a sampleholder; positioning the sample holder so that the ion beam is transverseto the vertical axis of the sample; milling the bottom side of thesample to remove at least of portion of the bottom surface of the sampleto produce a uniformly flat surface; positioning the sample holder sothat the bottom side of the sample is oriented toward the ion beamsource and so that the ion beam is parallel to the vertical axis of thesample; thinning the sample by directing the ion beam in a millingpattern that thins at least a portion of the sample to a thickness of 30nm or less.
 18. The method of claim 17 in which attaching the sample toa sample holder comprises: rotating the sample about an axisperpendicular to the sample vertical axis to invert the top and bottomsides of the sample; and attaching the inverted sample to a sampleholder.
 19. The method of claim 17 in which positioning the sampleholder so that the ion beam is transverse to the vertical axis of thesample comprises positioning the sample holder so that the angle betweenthe ion beam and the vertical axis of the sample is from 35 to 90degrees.
 20. The method of claim 17 in which thinning the sample bydirecting the ion beam in a milling pattern comprises thinning at leasta portion of the sample to a thickness of 15 nm or less.
 21. The methodof claim 17 in which milling the bottom side of the sample to remove atleast of portion of the bottom surface of the sample to produce auniformly flat surface comprises removing at least the bottom 25 nm ofthe sample.
 22. The method of claim 17 in which the substrate is asemiconductor wafer or portion thereof and the sample to be extracted isa portion of an integrated circuit that is to be observed using a TEM.23. The method of claim 18 in which extracting the sample from thesubstrate comprises attaching a microprobe to the freed sample andextracting the sample from the substrate using the attached microprobe,and in which attaching the sample to a sample holder comprises attachingthe sample to a sample holder and separating the microprobe from theattached sample.
 24. The method of claim 23 in which rotating the sampleabout an axis perpendicular to the sample vertical axis to invert thetop and bottom sides of the sample comprises inverting the sample byrotating the microprobe so that the orientation of the top and bottomsides is reversed.
 25. The method of claim 17 in which milling thebottom side of the sample to remove at least of portion of the bottomsurface of the sample to produce a uniformly flat surface comprisesmilling the bottom side of the sample using an ion beam having a firstaccelerating voltage and then milling the bottom side of the sample at asecond accelerating voltage, the second accelerating voltage being lessthan half of the first accelerating voltage.
 26. The method of claim 17in which milling the bottom side of the sample to remove at least ofportion of the bottom surface of the sample to produce a uniformly flatsurface comprises milling the bottom side of the sample using an ionbeam having an accelerating voltage of 5 kV or less.
 27. The method ofclaim 17 in which milling the bottom side of the sample to remove atleast of portion of the bottom surface of the sample to produce auniformly flat surface comprises milling the bottom side of the sampleusing an ion beam having an accelerating voltage of 30 kV or more. 28.An apparatus for carrying out the method of claim
 18. 29. Anon-transitory computer-readable storage medium configured with acomputer program, where the storage medium so configured causes acomputer to control a charged particle beam system to carry out thesteps of the method of claim
 18. 30. A method of preparing an ultra thinTEM sample for TEM analysis, the method comprising: loading a substrateinto an ion beam system including an ion beam source and optics forfocusing an ion beam along an axis and onto the substrate; freeing asample from the substrate by ion beam milling, said sample having avertical axis, a top side closest to the ion beam source, and a bottomside opposite the ion beam source; attaching a microprobe to the freedsample; extracting the sample from the substrate using the attachedmicroprobe; inverting the sample by rotating the microprobe so that theorientation of the top and bottom sides is reversed; attaching theinverted sample to a sample holder and separating the microprobe fromthe attached sample; positioning the sample holder so that the ion beamaxis is at an angle of 30 to 90 degrees relative to the vertical axis ofthe sample; removing at least 25 nm from the bottom of the sample toproduce a planar surface parallel to the ion beam axis; positioning thesample holder so that the top side of the sample is oriented oppositethe ion beam source and so that the ion beam axis is parallel to thevertical axis of the sample; thinning the sample by directing the ionbeam in a milling pattern that thins at least a portion of the sample toa thickness of 30 nm or less.
 31. An apparatus for preparing ultra-thinTEM sample comprising: an ion beam system including an ion beam source,optics for focusing an ion beam along an axis and onto a substrate, anda micromanipulator for manipulating a sample; and
 32. acomputer-readable memory storing computer instructions, the instructionsincluding a program for controlling the apparatus and causing theapparatus to carry out the steps of: (i) locating a desired sample siteon the substrate; (ii) separating a sample from the substrate by ionbeam milling; (iii) extracting the sample from the substrate, saidsample having a vertical axis, a top side, and a bottom side; (iv)attaching the sample to a sample holder; (v) positioning the sampleholder so that the ion beam is transverse to the vertical axis of thesample; (vi) milling the bottom side of the sample to remove at least ofportion of the bottom surface of the sample to produce a uniformly flatsurface; (vii) positioning the sample so that the bottom side of thesample is oriented toward the ion beam source and so that the ion beamis parallel to the vertical axis of the sample; and (viii) thinning thesample by directing the ion beam in a milling pattern that thins atleast a portion of the sample to a thickness of 30 nm or less.